import sys

from PyQt5.QtCore import QSize
from PyQt5.QtWidgets import QMainWindow, QPushButton, QApplication


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("My App")
        button = QPushButton("Press Me!")

        self.setFixedSize(QSize(400, 300))
        # Set the central widget of the window
        self.setCentralWidget(button)


app = QApplication(sys.argv)

window = MainWindow()
window.show()

app.exec_()